A joystick or an operation switch that is attached to a joint-driving motor unit fixed to the base end of a treatment instrument are known conventional examples of an operation input device for an electrical treatment instrument that has a joint at the distal end thereof and is introduced inside a body via a instrument channel of an endoscope (refer to PTLs 1 and 2, for example). The operation input devices described in PTLs 1 and 2 are fixed to the base end of the treatment instrument, and therefore, the operator operates the joystick while moving the elongated treatment instrument forward and backward in the longitudinal axis direction of the treatment instrument or while twisting the treatment instrument around the longitudinal axis using one hand holding the motor unit at the base end.